The present invention relates to a device for producing spunbonded nonwovens, i.e., flat webs formed by a plurality of continuous filaments. These filaments are formed by spinning or extruding molten plastics in devices containing a plurality of nozzles in which the plastic is shaped into filaments and discharged. The cohesion of the spunbonded nonwovens is explained by the more or less strong bonding of the filaments to one another at their points of intersection.
The present invention is concerned in particular with spinning nozzle packs for production of spunbonded nonwovens. These nonwovens have two continuous filaments that are different at least with respect to their melting range and are bonded by heat. These filaments are formed by simultaneously extruding two polymers with different melting ranges and then depositing them in an intermingled arrangement on a flat surface.
The resulting flat web is heated in the next operation to a temperature sufficient to soften only one type of filament, so that an adhesive bond is formed at all points of intersection of the two types of filament and at the points of intersection of the lower melting filaments with one another after cooling. The filaments with the lower melting range thus play the role of a binder.
Devices for producing spunbonded nonwovens are described in German Patent No. 34 19 675 C2. Spunbonded nonwovens are produced for use as carriers, optionally coated with bitumen, for all large-area sealing functions in the construction industry because of their stability. Their design is characterized by two types of thermally bonded continuous filaments. One type is formed by very high-melting polyethylene glycol terephthalate and is present in the nonwoven in the amount of 70 to 90 wt %, while the other type is formed by polybutylene glycol terephthalate and is present in the amount of 30 to 10 wt % and plays the role of the binder, because its melting point is only about 225.degree. C. For both types of filament, single filament titers of 4.5 to 6.5 dtex are reported.
The nonwoven is produced by extruding the two types of molten polymer for the respective filaments through spinning nozzles arranged side by side, with one spinning nozzle being assigned to one type of polymer physically and with regard to the material and temperature program. The spun filament bundles are drawn pneumatically from one side beneath the spinning nozzles and strike a baffle plate or guide plate which makes it possible to open the bundle. Then the filaments drop onto a continuous lattice apron. As an alternative, they can be combined and only then drawn together pneumatically. This yields an especially good and thorough mixing of the two types of filaments.
Continuous deposition is preferably followed by a needling operation and then thermal calendering, likewise performed continuously. The flat web then passes through the linear gap between two cylindrical rolls, at least one of which is heated. To do so, a temperature is selected that softens only the lower melting filaments to such an extent that they become capable of binding to the filament intersection points as described above. This is followed by an operation in which the solidified flat web is cooled between cooling cylinders and then wound up. In this stage, the finished nonwovens according to German Patent No. 34 19 675 C2 have a weight per unit area of 100 to 180 g/m.sup.2.
With the simultaneous extrusion of matrix filaments and binding filaments and processing according to the above teaching, flexible flat webs with a good dimensional stability are obtained. Integration of the operation of continuous thermal bonding permits economic production: thermal bonding reduces energy costs in production to about 1/8 in comparison with chemical bonding. According to German Patent No. 34 19 675 C2, the values for tensile strength and elongation at break in the various directions parallel to the plane of the nonwoven are close together.
These advantages must be seen against the requirement of having to use separate spinning nozzles with individual product and temperature programs for each type of filament (high melting matrix component, low melting bonding component). The space required for each individual nozzle, which has a lower limit because of the limited possibility of minimizing nozzle dimensions, also leads to a lower limit for the distance between the extruded filaments directly downstream from the nozzle outlet. Thus, there is also an upper limit for the specific throughput of spinnable material per unit of area of the device, i.e., within each spinning beam.
Especially when the filament titers differ greatly, the distance between matrix filament and binding filament, which cannot be further reduced, leads to fluctuations in their quantity ratios in the deposited nonwoven. This results in zones with higher and lower amounts of binding filament, which is reflected in corresponding unwanted fluctuations in the mechanical properties of the flat web. In this regard, the applicability of the teaching from German Patent No. 34 19 675 C2 is limited to matrix filaments and binding filaments of the same titer.